Synthesis of Whisker Reinforced Mullite Matrix Composites Using Colloidal Processing

Abstract

CVD-processed ultrafine mullite powder (<0.1m) micrometer size Si3N4-whisker or mullite-whisker were dispersed in H2-C2H5OH-C3H83 with a low sur face tension at pH 4.0 to prevent cracking during drying of wet green compacts consolidated by filtra tion. The rheological behavior of suspensions, consoli dation process of the solids by filtration, and the pore structures of green compacts were studied to under stand the formation of crack-free dense powder com pacts with uniformly dispersed whiskers in the nanom eter size matrix mullite. The crack-free powder com pacts were hot-pressed at 1500 for 1h at 10 to 39 MPa in N2 atmosphere or pressureless-sintered at 1600 to 1650 for 4 or 10h in air or vacuum. The monolithic mullite (99.9% T.D.) and 10% mullite whisker-added mullite (91.2% T.D.) with the average strengths of 468MPa and 242MPa, respectively, showed little degradation of strength at 1300. The composite with 10vol% Si3N4-whisker (99.8% T.D.) showed a high average strength (632MPa) at room temperature, while the strength decreased to 341MPa at 1300. The fracture toughness (1.68MPam0.5) of the monolithic mullite was increased to 2.94MPam.5 by the addition of 10% Si3N4-whisker.